We saw the scale of the Empire as a galactic entity with infrastructure and occupying forces as opposed to a military force that shows up to beat on the Rebels.
We know the Death Star was powered by kyber crystals looted from Jedi holy places, which is pretty fucking cool.
We saw a force user who wasn't a Jedi, and saw the force intervening in events where Jedis weren't involved, and we saw the Jedi home world/holy place.
We saw how the rebellion, literally days before A New Hope, was about to break apart and succumb to the Empire. In addition to getting the Death Star plans, them uniting in this movie was essential to their future victories.
We now know the exhaust port was a deliberate design flaw by a brave scientist rather than an inept one.
We understand the stakes going into a New Hope and the sacrifice the rebellion had recently paid to even get a slight - ahem - hope of defeating the Empire.
Sure, it's all additive rather than essential, but I think it adds so much richness to the universe and enhances rather than detracts from the original trilogy. We've especially never seen the Empire operating at its peak like this before, and the costs their grip on the galaxy had for the people living in it.